Delivery of very small amounts of liquids or gases is often of critical importance in research instrumentation. One way of dispensing an amount of liquid is to open a valve in a line containing fluid under pressure for a precise period of time. However, closely controlling the open time of an on-off valve in a flow line is not sufficiently accurate in many circumstances, because of variations in fluid viscosity, pressure differential and the like. A positive displacement metering system provides better control of volumetric flow rates. Representative prior positive displacement dispensers or metering devices are shown in U.S. Pat. No. 3,072,302, U.S. Pat. No. 3,353,712, U.S. Pat. No. 4,271,989, U.S. Pat. No 4,327,845 and U.S. Pat. No. 4,805,815.
U.S. Pat. No. 4,327,845 to Keyes et al. shows a dispensing apparatus having a rotary valve controlling access to a closed chamber containing a spring-loaded piston, acting as a fluid accumulator. A liquid, such as a viscous syrup or topping, under pressure, is admitted to the chamber, driving the piston upward, when the rotary valve is in one position; in a second position of the valve, the chamber is connected to an outlet, and the piston expels the contents of the chamber. A disadvantage of the keyes system is that the phase relationship between the inlet and outlet events cannot be changed. Also, there is no provision for changing the spring pressure on the piston, so as to obtain varying accumulator displacement.
U.S. Pat. No. 3,353,712 describes a dispensing system incorporating a fluid accumulator. The accumulator piston is downwardly biased by a spring centered on an adjusting screw which can be advanced to contact the back side of the piston physically, to limit its motion.
The use of pistons in very small displacement metering systems is problematic because of the exaggerated effects of any piston seal leakage. Furthermore, piston seals have drag which may be unacceptable for situations involving very low pressure fluid sources. Seal drag is also affected by the nature and temperature of the working fluid.